Solution: t 1 = 2. MCA = 2× 4.5 1s = 3s。

Let the resistance of MC segment be r, which is obtained from the law of electromagnetic induction and ohm's law of closed circuit:

0 ≤ t < 3s，i=BLatR+at2MNr=2t4+t2(A)。

When t≥3s, i=BLatR+2r=2t 13(A).

(2) According to (1), the time for the metal bar to reach the CD position is t 1 = 3s.

According to the ampere formula, FA = BIL = 2T4+T2.

According to Newton's law: F-FA-mgsinθ=ma.

Simultaneous solution: f =1.2+2t4+T2 =1.2+24t+t.

According to mathematical knowledge, when 4t = t, that is, t=2s, 4t+t has a minimum value, 24t+t has a maximum value, and force F has a maximum value, and because t = 2s < t 1 = 3s at this time, the problem condition is satisfied.

So the value of f at this time is the maximum value in this process, that is, FM =1.7n.

(3) Assume that the velocity of the metal rod drops to zero when it slides upward from the CD position for a distance s, and the initial velocity in this process is: v1= at1=1× 3m/s = 3m/s.

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